The present invention relates to a heat-sensitive recording paper, and more particularly, to such a heat-sensitive recording paper having very preferable recording aptitude which can be obtained by adding aluminum hydroxide into the color-developing layer of the heat-sensitive recording paper containing a coloreless or pale-colored chromogenic dyestuff and a phenolic substance as chromogenic elements.
A heat-sensitive recording paper that utilizes a heat color-developing reaction occurring between a coloreless chromogenic dyestuff having a structure such as a lactone, lactam, spiropyrane or the like and phenolic substance is disclosed, for example, in the Japanese Patent Publication Nos. 4160/68 and 14039/70, and the Japanese Laid-Open Patent Application No. 27736/73, and are now widely put into practical use.
Such a heat-sensitive recording paper is produced by applying on the paper surface the coating prepared by individually grinding and dispersing a coloreless chromogenic dyestuff and phenolic substance into fine particles, mixing the resultant particles with each other and then adding thereto a binder, filler, sensitizer, slipping agent and other auxiliaries. When subjected to heat, the coating undergoes a chemical reaction which instantaneously developes a color, and various colores can be advantageously developed in bright color depending upon selection of specific colorless chromogenic dyestuff.
These heat-sensitive recording papers have now been finding a wide range of applications, including medical or industrial measurement recording instruments, terminals of computer and information communication systems, printers of electronic calculators, facsimile equipment, automatic ticket vending machines, and so on.
These recording equipment has a heating element such as a thermal head or heating pen (stylus) and the heat-sensitive recording paper, when urged with such a heating element under predetermined pressure, is heated thereby to develop a color for recording.
Thermal heads that have been produced so far are diverse in their materials used and configuration. Accordingly, since the requirements for heat-sensitive recording paper vary largely with the performance (thermal response, thermal isolation among heaters, heat dissipation, heat resistance, wear resistance, power consumption, production cost), controlling method, recording conditions (impressed voltage, energizing time, surface temperature, contacting pressure, head scanning method, recording speed and contents of information to be recorded) or the like of the specific thermal heads used, the matching performances between the respective heat-sensitive recording papers and the recording equipments is very important. Especially in recent years, as the applications of recording equipment tend to be diversified and call for a higher performance, a higher quality has come to be required for heat-sensitive recording papers to be used thereon.
A thermal head is ordinarily subjected to its heating and cooling cycles repeatedly at a short cyclic period of 0.5-20 milliseconds, and the color-developing layer of a heat-sensitive recording paper contacted with the thermal head receives heat energy generated by the latter to cause a color-developing reaction for recording purpose. In this instance, in one type of a recorder, the thermal head itself laterally reciprocatingly moves to apply impressed pulses to a heating element at a timing required for a print image to thereby print images while feeding the recording paper, and in another type, the thermal head is stationary, and the heat-sensitive recording paper is fed at a timing of the impressed pulses.
In a heat-sensitive recorder such as an electrocardiograph, the heated stylus (heating pen) is contacted with the heat-sensitive recording paper under predetermined pressure, and is fluctuated while feeding the recording paper for recording.
The thermal head and the heating pen record while urging in contact with the heat-sensitive recording paper under predetermined pressure. Since the amount of heat conducted to the heat-sensitive recording paper depends greatly upon the close contact between the head or the pen and the recording paper, the heat-sensitive recording paper is desired to incorporate high smoothness on the surface thereof, and must have a sufficient color-developing sensitivity for producing bright chromogenic records with such a small heat input from the thermal head.
Inasmuch as the thermal head and the heating pen always make pressure contact and friction with the heat-sensitive recording paper, they are required to accommodate hard surface, wear resistance, and the heat-sensitive recording paper have to be less abrasive simultaneously.
The thermal head is constructed to normally have a wear resistant layer or resistor protective layer of hard material such as SiO.sub.2, Ta.sub.2 O.sub.5, Al.sub.2 O.sub.3, SiC, glass, etc. on a heating resistor such as Ta.sub.2 N, Ta-Si, Ta-Al, S.sub.n O.sub.2, Ni-Cr, silicone, RnO.sub.2, Pt-Ag, etc. However, the low cost thick film type thermal head incorporates no wear resistant layer sometimes. The heat-sensitive recording paper is strongly desired to be less abrasive for the thermal head when used for the recorder having such a thermal head.
Since the head pressure and sliding speed tend to be increased as the recent recording speed is accelerated and the packing image density is increased, the necessity for the low abrasiveness of heat-sensitive recording paper is further enhanced.
As the thermal head is, in addition, heated and cooled repeatedly in the recording process, heat-sensitive materials contained in the color-developing layer of the heat-sensitive recording paper is once melted and then solidified. In this course, a portion of the heat-sensitive materials may stick to the head surface and, consequently, the melted heat-sensitive materials may be accumulated on the head surface as "residues." If this occurs, such accumulated residues will obstruct the conduction of heat from the head to heat-sensitive recording paper to render printed images obscure and, sometimes, to such an extent that the records are hardly readable.
If the heat-sensitive paper also adhers or sticks to the thermal head causing a so-called "sticking," the movement of the paper or head will be obstructed with generation of offensive sounds and, in the worst case, the recording function itself may become impossible.
Further, as the heat-sensitive materials deposited on the thermal head may be retransferred to the surface of heat-sensitive paper, the thermal head may not be cooled sufficiently in the cooling cycle succeeding to the printing cycle due to the accumulation of the heat-sensitive material or an increase in the ambient temperature of the head may heat any portions of the heat-sensitive paper other than those required for intended printed images to cause undesired coloring thereat, the print image may be degraded with such phenomena as bleeding, smearing or ghost resulting therefrom.
Various addition materials are generally added to the coating to be applied on the color-developing layer of the heat-sensitive papers in addition to a chromogenic dyestuff, phenolic substance and binders constituting its basic ingredients. For example, with a view to improving the color-developing sensitivity, preventing adhesion of the heat-sensitive materials onto the head and preventing a frictional smudge, such waxes as paraffin waxes, polyolefin waxes, fatty amides and their methylol derivatives, higher fatty acids and their metal salts, condensates of a higher fatty acid and amine, polyhydric alcohol esters of higher fatty acids, higher alcohols and so on are added to heat-sensitive coating formulations.
Also, for improving the coating aptitude, whiteness and brightness, preventing adhesion of residues onto the thermal head, or preventing sticking of the heat-sensitive recording paper to the head, clays such as china clay, kaolin, talc, calcined clay, titanium oxide, calcium carbonate, magnesium carbonate, zinc oxide, etc. as fillers are used.
To obtain the heat-sensitive recording paper having superior recording aptitude to meet the requirements for high performance and recording speed of recent heat-sensitive recording equipments, the aforementioned various addition materials must be suitably mixed. However, the kaolin and other fillers heretofore used exhibit a problem in the abrasiveness for the thermal head and the heating pen.